The present disclosure relates to infrared detectors, and more particularly to pyroelectric infrared detectors outputting balanced signals.
A pyroelectric infrared detector includes a pyroelectric element made of a pyroelectric material. The pyroelectric material is one type of dielectric material. If the temperature of the pyroelectric material itself changes in accordance with a change in energy of infrared light incident on the pyroelectric body, the amount of charge (induced charge) on both of a front and back surfaces of the pyroelectric body is changed. If electrodes are provided on the both surfaces of the pyroelectric body, and a load is connected between the electrodes, the induced charge flows through the load as a current (a pyroelectric current). Voltage mode operation by high load resistance connected between electrodes, and current (charge) mode operation for detecting a pyroelectric current by low load resistance are generally used in a pyroelectric infrared detector.
Pyroelectric infrared detectors are widely used as detectors in proximity sensors for human body detection. In general, a human body is considered as a thermal radiation source of about 310 K. If a pyroelectric element has an area of several mm per side, a change in input energy to the pyroelectric element due to the human body is considered to be about 10 μW. If a conventional pyroelectric element made of a pyroelectric material such as lead titanate (PbTiO3) or lead zirconate titanate (PZT), the amount of charge generated in the pyroelectric element changes by about 0.1 pC with the change in the input energy of about 10 μW. The change in the charge amount is about 1 mV, if calculated in terms of a change in a voltage between two electrodes of the pyroelectric element. As such, a signal generated in the pyroelectric element is extremely weak. Therefore, pyroelectric infrared detectors are required to have good resistance to external noise. As major external noise, noise introduced into a pyroelectric element and a signal extraction circuit, and noise introduced into a transmission path of the extracted signal are considered.
As a method of extracting a signal from the pyroelectric element, there is, for example, a method of converting to a voltage, a pyroelectric current being a time rate of an induced charge using a feedback capacitor of an amplifier. The reason for this is considered to be that a capacitor of the pyroelectric element and the feedback capacitor amplify the induced charge. There is also a method of connecting a high-resistance resistor to a pyroelectric element to extract voltages at both ends of the resistor as signal voltages via a source follower buffer of a FET (see, e.g., Japanese Patent Publication No. H10-281866).